Drawing a parallax background using a fragment shader.

parallaxShaderF.cpp

#include <SFML/Graphics.hpp>

int main()
{
    sf::RenderWindow window(sf::VideoMode(1024, 768), "Parallax Example",
                            sf::Style::Titlebar | sf::Style::Close | sf::Style::Fullscreen);
    window.setVerticalSyncEnabled(true);

    sf::Texture texture;
    // This is a texture 1024x1024 with three backgrounds stacked one over the other from front to back
    if (!texture.loadFromFile("resources/cave.png"))
        return EXIT_FAILURE;
    texture.setRepeated(true);
    sf::VertexArray sprite(sf::TriangleFan);
    sprite.append(sf::Vertex(sf::Vector2f(0,0), sf::Vector2f(0,0)));
    sprite.append(sf::Vertex(sf::Vector2f(1024, 0), sf::Vector2f(1024, 0)));
    sprite.append(sf::Vertex(sf::Vector2f(1024, 768), sf::Vector2f(1024, 1024)));
    sprite.append(sf::Vertex(sf::Vector2f(0, 768), sf::Vector2f(0, 1024)));

    sf::Shader parallaxShader;
    parallaxShader.loadFromMemory(
        "uniform sampler2D texture;"
        "uniform float offset;"

        // This shader is just for demonstration purposes. There's most likely a better way to do it. :)
        "void main() {"
        "    vec2 p3 = gl_TexCoord[0].xy; p3.y = p3.y / 3.;"
        "    vec2 p2 = p3; p2.y = p2.y + .33; p2.x = p2.x + offset / 10;"
        "    vec2 p1 = p3; p1.y = p1.y + .67; p1.x = p1.x + offset / 100;"
        "    p3.x = p3.x + offset;"
        "    vec4 bg1 = texture2D(texture, p1);"
        "    vec4 bg2 = texture2D(texture, p2);"
        "    vec4 bg3 = texture2D(texture, p3);"
        "    bg1 = bg1 * (1 - bg2.a) + bg2 * bg2.a;"
        "    gl_FragColor = bg1 * (1 - bg3.a) + bg3 * bg3.a;"
        "}"
        , sf::Shader::Fragment);

    float offset = 0.f;

    sf::RenderStates state;
    state.blendMode = sf::BlendAlpha;
    state.shader = &parallaxShader;
    state.texture = &texture;

    sf::Clock clock;
    while (window.isOpen())
    {
        sf::Event event;
        while (window.pollEvent(event))
        {
            switch (event.type) {
            case sf::Event::Closed:
                window.close();
                break;
            }
        }

        // I set an arbitrary value as the offset, you'd calculate this based on camera position
        parallaxShader.setUniform("offset", offset += clock.restart().asSeconds() / 2);

        window.clear();
        window.draw(sprite, state);
        window.display();
    }

    return EXIT_SUCCESS;
}